The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the production of gradient fields used in imaging and spectroscopy pulse sequences.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B,) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as "spin-warp". The spin-warp technique is discussed in an article entitled "Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
The imaging gradients are produced by gradient amplifiers that drive coils which produce magnetic fields having gradients directed along physical axes. Typically, three gradient amplifiers with corresponding coils produce gradient fields directed along three orthogonal axes, x, y, and z. These are "physical" axes because they are fixed with respect to the MRI system geometry.
MRI pulse sequences are defined by the gradient fields and the RF fields that are to be produced during each NMR measurement. The gradient fields are defined by gradient waveforms produced along three orthogonal axes. For example, a slice-selecting gradient may be applied along one axis; a phase encoding gradient may be produced along another, orthogonal axis; and a readout gradient may be produced along yet another orthogonal axis. These "logical" axes may correspond to the physical axes on the MRI system for one orientation of the slice or volume to be images (e.g. a transverse plane), but as a general matter they do not. When the MRI pulse sequence is performed on an MRI system, therefore, each logical gradient field may be produced by the combination of one to three of the physical gradient fields, depending on the prescribed orientation of the image to be acquired.
When a pulse sequence is executed during a patient scan the logical gradient waveforms are converted into physical gradient waveforms for driving the gradient amplifiers on the MRI system. This is performed by a matrix rotation of the logical gradient waveforms. As described, for example, in U.S. Pat. No. 4,743,851, the logical gradient is stored as a series of points which represent the amplitude of the gradient at successive time increments (e.g. every 4 microseconds) during the pulse sequence. These points are read out of memory in sequence and multiplied by three rotation factors to rotate the logical gradient into each of the three physical axes. For three logical gradient waveforms, this involves nine multiplications and additions for every time increment of the pulse sequence.
One of the practical limitations of MRI systems is the heat dissipation capacity of gradient amplifiers and coils. Pulse sequences can often be shortened to reduce total scan time by increasing the amplitude of a gradient. Unfortunately, higher amplitude gradients produced at a higher repetition rate also produce more heat. As a result, gradient heating often limits the extent to which a pulse sequence can be shortened.
Prior to the commencement of a scan it is desirable to calculate the amount of gradient heating that will occur to insure that limits are not exceeded. Since the logical gradient waveforms are stored as a series of amplitude points, it is relatively easy to integrate the logical waveforms over the duration of the pulse sequence and determine the amount of heat each will produce. This does not necessarily reveal, however, how much heating will occur in the physical amplifiers and coils because the waveforms may be changed considerably during the rotation into the physical axes. The computations necessary to rotate the logical gradient waveforms and calculate heating in each physical axis is substantial and not usually performed in state-of-the-art MRI systems. As a result, gradient pulse amplitudes are often unnecessarily derated to insure that heating problems do not occur.